Commentary: The trouble with data

   While listening to the very informative State of Freight webinar from transportation analyst FTR Associates in early November, there were some interesting statistical conundrums for senior analysts Noel Perry and Larry Gross.
   Gross first noted the intermodal figures from the Intermodal Association of North American (IANA) for September didn’t jive well with figures from the American Association of Railroads (AAR).
   “There’s murkiness in the intermodal data,” Gross said. “The two sources are pointing to opposite things happening in September.”
   IANA’s data showed intermodal volume growth for the month down 1.7 percent, while AAR noted it up 5 percent. In terms of actual volume, the difference reported between the two organizations was 9.7 percent. 
   “To be frank, we have no explanation for the divergent figures,” he said, explaining that there is typically about a 5 percent gap between their actual volumes, since a handful of regional railroads don’t report to IANA.
   Gross said the gap in reported volumes narrowed for the full third quarter (about 7 percent).
   “These variances settle down over time,” Gross said. “There’s lots more volatility in the IANA numbers, so we’re taking these with a grain of salt and expect October numbers to even some of this out.”
   Meanwhile, on the trucking side, a similar gap emerged between FTR’s growth metrics and those provided by the American Trucking Associations and Cass, a freight payment vendor that generates its own index on the industry.
   FTR’s third quarter metrics had trucking growth at nearly 5 percent in the third quarter, while ATA’s index was at 1.5 percent and Cass’ at below 1 percent.   
   Perry called the gap a “disconnect.”
   “The people that generate freight for the trucking industry are pretty healthy,” he said. “So the manufacturing sectors that feed us freight are OK. But the ATA and Cass numbers are flat. The customer base is healthy, but something is keeping that health from flowing to who reports to ATA and Cass.”
   For those of us who rely on numbers and the organizations that synthesize data points such as this, it’s a confusing situation when two measures of the same subject produce drastically different interpretations.
   But it’s also a healthy lesson for us not to rely wholly on one set of numbers or another. Divergent data can sometimes tell us there’s more to a story than one index or a survey may illuminate. — Eric Johnson